1. Field of the Invention
The present invention relates to a magnetic toner, and to a developing apparatus and an image forming apparatus using such a magnetic toner.
2. Description of the Prior Art
Toners for use in the development of electrostatic latent images are roughly grouped into non-magnetic and magnetic toners. In image development, non-magnetic toners are usually used in the form of two-component developers, i.e., mixed with a carrier. Two-component developers maintain the electric charge of the toner stably, and thus offer satisfactory images for an extended period. However, they are susceptible to variations in the mixing ratio between the toner and the carrier and to deterioration of the carrier.
On the other hand, magnetic toners are usually used singly, i.e., in the form of one-component developers. One-component developers, containing no carrier, permit size and weight reduction of developing apparatus, and do not require maintenance such as replacement of the carrier. For these reasons, in recent years, magnetic toners have come to be used not only in low-speed, small-size copiers and printers but also medium- and high-speed copiers and printers.
Magnetic toners usually do not contain colorants, and their colors originate from magnetic powders. However, in general, magnetic powders are not purely black but reddish, making images developed with a magnetic toner appear reddish. That is, magnetic toners do not always offer a satisfactory tone of color.
The easiest way to alleviate this reddishness of images formed with a magnetic toner is to reduce the magnetic powder content of the toner. However, reducing the magnetic powder content of a toner results in reducing the coercivity of the toner, and thus leads to disturbance of a thin toner layer on a developer supporting member (hereinafter referred to as a “developing sleeve” also) and to increased toner adhesion in a non-image area (fogging).